A few notes about handhelds
As a limited amateur radio operator, I have an Icom T7-H that has a max output of 6 watts. This is when it is connected to an external power source of 11.5 - 14.5 volts DC (i.e. the power point in my truck) and the ouput switch is set to "high". With just the rubber duck antenna, I can successfully reach a repeater station over 15 miles away. When I connect the external antenna, I can do the same at half power output (about 3 watts) or with just running on batteries and max power output. All this is on the ground (relatively flat ground with buildings no higher than 5 floors).
Take that same handheld in the plane and, depending on the weather, I can reach repeater stations over 30 miles away due to the "line of sight" being better and no ground obstacles in the way. I haven't tried it with stations that are more than about 30 miles away yet.
Regarding batteries, NiCd batteries are better than NiMH because they will maintain a constant level of power until they are close to being drained. NiMH batteries will loose available power over the "drain life".
As for connecting the unit to a headset, that will cause more power drain than turning up the volume.
Bottom line is this... A handheld in an aircraft is an excellent idea for backup/redundancy. If you can connect it to a power point in the aircraft and the aircraft's electrical system hasn't gone north (I'm in the south

) and/or an external antenna, you're better off but even just as a bone stock standalone, it's still pretty worthwhile.
The comment about losing navs with comms is pretty accurate since most of the time they are the "same" unit in smaller GA aircraft (i.e. - most singles and light twins) and run off the same power connection.
Keep in mind that you could call another aircraft and have them relay the message to ATC/other ground units too.